The present invention relates to improvements in human tissue stimulators and more particularly to a human tissue stimulating system which in a preferred form comprises an audio responsive system for artificially stimulating the cochlea to improve the hearing of the hearing impaired.
U.S. Pat. No. 4,400,590 issued Aug. 23, 1983 for "Apparatus for Multi-Channel Cochlear Implant Hearing Aid System" describes and illustrates a multi-channel intra-cochlear electrode and system for electrically stimulating predetermined locations of the auditory nerve within the cochlea of the ear. The electrode comprises a plurality of exposed electrode pairs spaced along and embedded in a resilient curved base for implantation in accordance with the method of surgical implantation described in U.S. Pat. No. 3,751,615 issued Aug. 7, 1973 for "Method of Inducing Hearing." The hearing aid system described in the '590 patent receives audio signals at a signal processor located outside the body of a hearing impaired patient. The processor converts the audio signals into analog data signals which are transmitted by a cable connection through the patient's skin to the implanted multi-channel intra-cochlear electrode. The analog signals are applied to selected ones of the plurality of exposed electrode pairs included in the intra-cochlear electrode to electrically stimulate predetermined locations of the auditory nerve within the cochlea of the ear where the intra-cochlear electrode is positioned.
The cochlea stimulating system described in the '590 patent is limited in the number of channels of information, the speed of transfer of stimulating signals to the cochlea and the fidelity of the signals. Also, the cable connection through the skin of the patient to the intra-cochlear electrode is undesired in that it interferes with the freedom of movement of the patient and represents a possible source of infection.
U.S. Pat. No. 4,532,930, issued Aug. 6, 1985 for "Cochlear Implant System For an Auditory Prosthesis" describes and illustrates a multiple electrode system. While multiple electrodes are employed to stimulate hearing the system only operates with a single pulsatile output stimulating a single electrode channel at any given time. Such a sequential system is limited in speed of operation and does not provide for analog operation where continuous stimulating signals controllable in amplitude are simultaneously applied to a number of electrode channels. Further, the system is subject to charge imbalance with misprogramming or circuit fault and inefficient use of electrical power. Moreover, once the stimulator unit is implanted there are no means for monitoring its ongoing circuit operation or power requirements so as to optimize its continued operation.
U.S. Pat. No. 4,592,359, issued Jun. 3, 1986 for "Multi-Channel Implantable Neural Stimulator" describes a cochlear implant system having 4 current sources and 4 current sinks per channel controlled by series switches to provide 16 different circuits for supplying 16 levels of 2 polarities to each output channel. In a pulsatile mode, the system provides for simultaneous update (amplitude control) and output to all channels. However, the system does not permit simultaneous analog update and output on all channels and the electrode pairs for each channel are not electrically isolated from all other electrode pairs whereby undesired current leakage may occur. Further, once the stimulator is implanted there are no means for monitoring its ongoing circuit operation or power requirements so as to optimize its continued operation.
U.S. Pat. No. 4,947,844, issued Aug. 14, 1990 for "Receiver/Stimulator For Hearing Prosthesis" describes and illustrates a multiple channel electrode system. The system includes an implanted receiver/stimulator connected to an implanted electrode array. The receiver/stimulator includes an electrode stimulating current control characterized in that current is delivered to each electrode or to each bipolar pair of electrodes in a series of short electrical pulses, each elemental pulse being separated from the next by an interval of zero current which has a longer duration than an elemental pulse. The waveform of the stimulus current comprises a series of pulses of one polarity followed by an equal number of pulses of an opposite polarity whereby the sum of all the electrical charge transferred through each electrode is approximately zero at the end of a stimulating current waveform. In this way, elemental current pulses applied to each electrode on each pair of electrodes which are stimulating are preferably delivered cyclically such that elemental pulses delivered to one electrode are interleaved in time with those delivered to any other electrodes. This enables the use of a single current source in the receiver/stimulator. The use of a single current source limits the operation of the receiver/stimulator in that the single current source must be switched to serve all output channels in a sequential manner. Simultaneous operation is not possible. Further, the number of channels cannot be greater than 3 or 4 without greatly reducing the duty cycle of the stimulating current waveform in each channel. Not only does the stimulus effectiveness in each channel suffer in such a situation, but the time required to complete the switching cycle for the single current source lengthens in direct proportion to the number of channels. Further, this system lacks output coupling capacitors in series with each electrode. This omission may lead to net DC current flow through the electrodes in the event of misprogramming or under circuit fault conditions.
The system described in the '844 patent also includes in the implanted receiver/stimulator a transmitter for telemetering one electrode voltage, measured during stimulation, to an external receiver for monitoring and analysis as an indicator of proper operation of the implanted stimulator. The transmitter comprises an oscillator operating at a frequency of about 1 MHz. The output of the oscillator is coupled to the implant's receiving coil and demodulated to recover the selected voltage waveforms. Unfortunately, such a telemetry system is not only limited to the monitoring of one voltage, but the simultaneous transmission of the telemetry signal and reception of the input carrier signal as described will result in undesired modulation and possible loss of input data.
For cochlear stimulator applications, it may be desirable to employ a cochlear stimulator that is driven by a behind-the-ear speech processor, e.g., of the type described in U.S. patent application Ser. No. 08/807,734, filed Feb. 28, 1997. Behind-the-ear speech processors offer several advantages, but require very low power dissipation. Although low power digital electronics have enabled digital hearing aids, this technology is only part of the answer for implantable stimulators. This is because an implantable stimulator, e.g., a cochlear stimulator, requires a fixed current to stimulate the target tissue, e.g., the auditory nerve within the cochlea, and this power must be transferred across a transcutaneous link, that is, at best, only about 50% efficient. While digital hearing aids only directly need to drive a transducer that uses less than one milliwatt (mW) of power, an implantable tissue stimulator may require up to 50 mW of stimulus power, which means (assuming a 50% transcutaneous link transfer efficiency) the need to transmit up to 100 mW of power to the implant device. Since power is proportional to the square of the voltage, it would thus be desirable to have a way to precisely and actively control the voltage in the implant device to track the output power requirements of the device. For example, for a cochlear stimulator, where room sound and speech levels are variant, it would be desirable to track speech and system variations and make automatic adjustments in the input power that track these variations, thereby only transmitting power to the implant device that is needed for the current conditions, thereby increasing the life of the battery.
Accordingly, there is a continuing need for an improved multi-channel tissue stimulator system particularly useful as a cochlear stimulator system and which is characterized by: (i) a high operating speed in analog and pulsatile operation, (ii) freedom from charge imbalance, (iii) complete isolation of its electrode pairs for each channel, and (iv) the externally controllable monitoring and selective control a plurality of operating parameters and power supply to and currents and voltages developed within the implanted stimulator unit of the system to optimize system operation and power efficiency. The present invention satisfies such needs.